There is a need for a low cost intelligent public pay telephone. It is desirable that such a telephone be reliable and durable in light of the hostile conditions and environment within which the telephone may have to operate. It is further desirable that this telephone be rich in features that will assure reliable performance.
a) In particular, there is a need to reduce input/output (I/O) pin count circuits used within the present invention for the purpose of incorporating other features requiring I/O reducing cost of circuit functions and improving product reliability.
b) There is a need to reduce microprocessor "bit-banging" (a dedicated CPU-intensive control technique that doesn't allow for simultaneous CPU functionality) when communicating with serial devices used within a telephone system, and to facilitate a reduced-power, high speed method of downloading or uploading pay station data, records, and operating program code, while reducing the product cost and improving product reliability in terms of printed circuit board device interconnections.
c) There is a need to promote software security and to provide a technique of controlling product firmware releases on a per phone basis, to provide unique pay station operation, and to facilitate future firmware or software control on a per phone basis. There is further a need to define a method of controlling firmware configurations.
d) A low power (microamp range), low impedance hook switch interface circuit is needed which is immune to the affects of intruding water, or any other input impedance reducing agent or effect that can cause a false hook switch event or indication. In addition, a method of verifying a circuit oscillator operation (32.768 kHz in this case) in terms of frequency and duty cycle is needed.
e) There is a need to interface the phone chassis with various mechanical coin acceptors that generate a wide range of nickel, dime, and quarter coin pulse widths utilizing a uniform detection algorithm for all coin acceptors. In order to reliably detect the various pulse widths, there exists a need to establish a minimum valid pulse width criteria, and to reject any noise associated for a given coin acceptor mechanism. There is an added need to make coin input filtering less of a CPU intensive function.
f) It is desirable to have a flexible and secure memory page-swapping technique that allows a 64 kbyte direct address range to be expanded, and then be logically repartitioned in a number of segments such that multiple devices in a system can be partitioned in such a way as to not conflict with each other, and further to write-protect all memory on a segment resolution basis.
g) The ability of determining the condition of the phone's primary battery for the purpose of assuring proper phone operation of future battery-dependent functions.
h) There is a need to prevent pin fraud on coin telephone trunk applications.
Pin fraud is a common method of defrauding the telephone. One pin fraud technique includes inserting a fine pin into any one of the four handset interconnect wires within an armored handset cord (provided the interconnect is not ac coupled or isolated in some way) and shorting the pin to the metallic armored cord which is connected to earth ground. This event must take place at the time the central office is performing a local call coin check. Shorting a +5V referenced signal to earth ground will complete the circuit loop, and as a result be interpreted by the central office that the minimum deposit has been satisfied, thereby enabling the call. Another pin fraud technique includes inserting a pin into the armored handset cord as described previously at the time of a post call collect signal application, thereby diverting the collect signal from the escrow relay. Then, upon removing the pin and coming off-hook and going back on-hook, which prompts the central office to send a refund signal to the phone, any money held in escrow from a long distance coin call will be returned instead of collected.